1. Field of the Invention
The present invention relates to a diffractive optical element for producing diffracted light flux relative to an incident light, and more particularly to a diffractive optical element capable of being used in wideband wavelength.
2. Description of Related Art
A diffractive optical element is an optical element having a lattice structure of slits or grooves spaced at even intervals with several hundred lines per a small distance (about 1 mm), and it has the characteristic that when a light is incident to it, it produces diffracted light fluxes in directions determined by the wavelength of the light and the separation (pitch) of slits or grooves. Diffractive optical elements like this are used in various kinds of optical systems. For example, an optical element used as a lens for converging a specific order of diffracted light into a point has been known.
Among known diffractive optical elements, a so-called multi-layer diffractive optical element has been proposed. The diffractive optical element of this type has structure laminating a plurality of diffractive elements having a saw-tooth shape surface in a form appressed with each other. It has a characteristic of high diffractive efficiency over almost the entirety of a required wide wavelength range (for example, whole visible light range from g-line (435.8 nm) to C-line (656.3 nm)) or, in other words, good spectral characteristic.
Generally, a multi-layer type diffractive optical element has one of a laminated multi-layer structure constructed by a glass material 60 cemented with a resin material 70 at the same diffraction grating 100 as shown in FIG. 3A, or a separated multi-layer structure constructed by a first diffractive optical element 160 having a first diffraction grating 110 and a second diffractive element 170 having a second diffraction grating 120, in which respective diffraction gratings 110 and 120 are closely located and facing with each other, for example, respective gratings facing each other separated by an air space 130 as shown in FIG. 3B. Here, in the case of a separated multi-layer type diffractive optical element, in order to satisfy the condition for correcting chromatic aberration at two predetermined wavelengths, the groove height d110 of the first diffraction grating 110 of the first diffractive element 160 is set to a predetermined value and the groove height d120 of the second diffraction grating 120 of the second diffractive element 170 is set to another predetermined value. Accordingly, diffraction efficiencies for the predetermined two wavelengths become 1.0 and high diffractive efficiency can also be obtained at the other wavelengths. In a transparent type diffractive optical element according to the present specification, diffractive efficiency is defined as a ratio η (=(I1/I0)×100%) of intensity of a first order diffracted light I1 to that of an incident light I0.
In the multi-layer type diffractive optical element with the above-described construction, although diffraction efficiency can be made high within wide wavelength range, it has a problem that a rate of decrease in diffractive efficiency relative to variation in the incident angle of the incident light (hereinafter called angular characteristic) becomes worse in comparison with a single-layer type diffractive optical element as shown in FIG. 4B. In a single-layer type diffractive optical element, the incident angle at which diffraction efficiency becomes lower than 90% when the pitch of the diffraction grating grooves is about 0.1 mm is about 30 degrees. On the other hand, in a multi-layer type diffractive optical element, the incident angle at which diffraction efficiency becomes lower than 90% when the pitch of the diffraction grating grooves is about 0.1 mm is about 10 degrees.
The reason why angular characteristic of a multi-layer type diffractive optical element is worse than that of a single-layer type diffractive optical element is the difference in the wall height of each diffraction grating groove. The wall height d20 of the diffraction grating groove 20 of the single-layer type diffractive optical element shown in FIG. 4B is about 1 μm. On the other hand, the wall height of the multi-layer type diffractive optical element becomes more than 10 μm. In particular, the wall height d10 of the diffraction grating groove 10 of the multi-layer type diffractive optical element shown in FIG. 4A becomes more than 20 μm.
Due to the difference in wall height, respective areas r and r′ where incident light proceeds in the diffraction grating with a given optical path among all incident light having an incident angle of α shown in FIG. 4 differ with each other. The area r corresponding to the multi-layer type diffractive optical element shown in FIG. 4A is narrower than the area r′ corresponding to the single-layer layer type diffractive optical element shown in FIG. 4B. So the angular characteristic becomes worse in the multi-layer type diffractive optical element.
Among all incident light, the light rays passing through the wall of the diffractive optical element (for example, the wall 30 of the multi-layer type diffractive optical element shown in FIG. 4A) become unnecessary light (hereinafter called flare), which does not follow the designated optical path to reach a designated position. As the angle of incidence increases, the amount of flare increases, so the optical performance of the diffractive optical element becomes worse. The produced flare becomes an ordinary reflected light including a total reflection light (case A) or scattered light (case B) in accordance with the state of the wall surface 30. The area fL producing the flare becomes narrow as fL′ when the wall height of the diffraction grating groove is low as shown in FIG. 4B, such that flare is scarcely produced.
In a method for reducing the flare of the diffractive optical element described above, there is a construction having a shade for blocking a light incident to (or leaving from) the wall of the diffraction grating groove locating on the incident (or exit) plane of the diffractive optical element. This is disclosed on page 3 in FIG. 1 in Japanese Laid-Open Patent Application No. 2002-48906.
However, when a construction forming a shade on the incident (or exit) plane of the diffractive optical element is applied, the shade must be formed on the position of the incident (or exit) plane exactly correspondent to the wall of the diffraction grating groove, so it is difficult for the diffractive optical element to be manufactured.